Design Manual

8 R-TRWH-E-02 Rev D
Pump Sizing for Circulation
1. Use the chart below or one appropriate for your conditions to determine the heat loss in the length of the hot water
supply and return piping. For example, 100 ft of 1-1/2 in bare copper tubing results in a heat loss of 5300 Btu/h.
2. Determine the acceptable temperature drop at the last fixture in the loop. For example, if the supply temperature from
the water heater is 120 ºF (49 ºC) and an acceptable temperature at the last fixture is 100 ºF (38 ºC) then the
acceptable temperature drop is 20 ºF (7 ºC).
3. Calculate the required pump flow rate using the following formula:
FLOW RATE (gpm) = HEAT LOSS (BTU / h)
500 X ACCEPTABLE TEMPERATURE DROP (ºF )
4. Based on the above calculations select a pump for the type of circulation system you will be utilizing:
A). Optional Method (reference drawing WH1-RGE) - Reference pump manufacturers flow vs. pressure
specifications to select a pump that can provide the flow rate calculated above while overcoming the
pressure loss through:
• Tank water heater (reference manufacturer’s information)
• All building supply and return plumbing in the circulation loop (reference local plumbing codes,
standards, or practices)
B). Preferred Method (WH1-CD-O, WH1-CD) - Reference pump manufacturers flow vs. pressure specifications
to select a pump that can provide 3 gpm of flow or the flow rate calculated above, whichever is greater, while
overcoming the pressure loss through:
• Rinnai tankless water heater (reference flow vs. pressure curve of the Rinnai model being used)
• Optional storage tank (reference manufacturer’s information)
• All building supply and return plumbing in the circulation loop (reference local plumbing codes,
standards, or practices)
NOTE: Only use pumps of brass, bronze, or stainless steel construction. Do not use pumps of iron construction as
they will oxidize and clog the inlet filter on the appliance. The pump should be controlled by an aquastat,
timer, or combination of the two. A demand control (motion sensor, push button, or door contact) may also
be used.
Nominal Size, in. Bare Copper Tubing,
Btu/h-ft
1/2 in. Glass Fiber Insulated
Copper Tubing, Btu/h-ft
3/4 30 17.7
1 38 20.3
1-1/4 45 23.4
1-1/2 53 25.4
2 66 29.6
2-1/2 80 33.8
3 94 39.5
4 120 48.4
Approximate Heat Loss from Piping at 140 ºF Inlet, 70 ºF Ambient *
* Source: 2011 ASHRAE Handbook HVAC Applications